U.S. intelligence officials said Friday the situation in western Iraq was "extremely dire" after radical Sunni forces linked with al Qaeda raised their flag in the town of Fallujah - site of two of the bloodiest battles during the Iraq war - and gained control of the city.
Islamist insurgents have also battled tribesmen for control of the Iraqi city of Ramadi.
The fighters brandished their weapons and set police vehicles ablaze on Wednesday, The Associated Press reported. A provincial spokesman said the militants had taken over police stations and military posts in Fallujah and Ramadi after security forces left.
An interior ministry official told Agence France-Presse that ISIL, the al Qaeda-linked Islamic State of Iraq and the Levant, remained in control of parts of the two cities on Thursday.
The move is another sign that Prime Minister Nuri al-Maliki has not been able to maintain control of the country since the United States withdrew its troops in 2011, failing to reach an agreement with the Maliki government to leave behind a residual force.
There are currently no U.S. troops in Fallujah or Ramadi, according to the Pentagon.
The State Department said the violence that reached another peak Friday was a spillover from the civil war in neighboring Syria -- and that the border between Syria and Iraq has now become meaningless.
The United States rejected suggestions that American troops could help stabilize the situation.
"If we couldn't control that border with 150,000 troops in that country during the war, what would a few hundred accomplish?" one U.S. official said.
The U.S., however, did last month authorize the sale to the Maliki government of Hellfire missiles and Scan Eagle surveillance drones.
The United States continues to have a large diplomatic presence in the country, along with about 100 Marines and 100 high-level institutional trainers -- nearly all in Baghdad. There is also a smaller U.S. Consulate in Erbil.
During the two battles of Fallujah in 2004, the U.S. lost 51 and 95 troops, respectively. More than 1,000 U.S. troops were injured in total.
NBC News' Becky Bratu contributed to this report.